The major nonglycosylated structural proteins of mammalian type C RNA tumor viruses are initially synthesized in the form of a 65,000 dalton gag gene-coded precursor. In the murine system the internal sequence from the 5' to the 3' end of the gag gene has been established as p15-p12-p30-p10. Analysis of cell lines nonproductively-transformed by various mammalian sarcoma virus isolates has revealed expression of different numbers of type C viral proteins occurring progressively from the 5' to the 3' end of the gag gene. In cells transformed by feline sarcoma virus, or either of three replication-defective transforming virus isolates of murine origin, the amino terminal gag gene-coded proteins are initially expressed in the form of 100,000-130,000 molecular weight precursors. In each case these sarcoma virus-coded precursors have been shown to lack immunologic cross reactivity with p30, p10 or gp70 and in the feline system a 60,000 molecular weight cleavage product of the FeSV-coded precursor has been shown to contain transformation-specific immunologic determinants. These findings raise the possibility that these transforming viruses represent genetic recombinants between leukemia virus genomes and cellular sequences whose activation is involved in the etiology of both spontaneous and chemically-induced tumors.